The instrument, invented by the Dutch mathematician and physicist Willem Jacob's Gravesande (1688-1742), a professor at the University of Leyden, is made up of a vertical metal staff with a sliding ring and a metal ball suspended over an alcohol burner.Also known as Gravesande's ring, its purpose is the show that solids expand when they are heated. When the sphere, suspended from the top of the shaft, is at room temperature, it passes freely through the ring. When sufficiently heated it can no longer pass through it.The instrument on exhibition at the Museum consists of a brass staff (height 33 cm) fixed to a wooden base (width 8 cm, depth 16 cm); the brass ring is 3.05 cm in diameter and the sphere is 3 cm.